Hey guys,

New to the forum - glad to be part of the team!

I just picked up my first Ford truck - 1998 F250 Lariat 4x4 with just shy of 150k miles. Unknown maintenance history.

So far the truck is great, but I'm having one little issue that is driving me up the wall. Not really having any luck searching for a solution so maybe some experts can chime in?

I've got a noise... A metal-on-metal noise that I can hear coming from the underside of the truck. The noise starts around 30mph and increases in frequency as speed increases. If I take my foot off the gas, the noise stops. As soon as I depress the throttle even just slightly, the noise returns. Turning does not affect the noise and below about 30mph there is no noise present at all regardless of throttle position.

The noise is related to vehicle speed, not engine speed (i.e. RPM does not change the frequency but speed does).

The noise is like if you took two pieces of sheet metal and rubbed them together very quickly. It's definitely from something rotating. It doesn't sound like the typical wheel bearing noise, and I'm struggling with the fact that depressing the accelerator or not makes a difference.

Any help would be greatly appreciated - thanks in advance!

 

drive shaft U-joints, rear pinion bearing ?

 

Pinion bearing sounds like a winner, little bit of play in the driveshaft at the pumpkin. What is the bearing called at the front of the driveshaft (at the transfer case I believe?)? Output shaft bearing? There also seems to be some play here too, more than in the rear (HA!)

Are these things a DIYer can accomplish on a weekend without too much swearing, or just get it to a shop?

 

clearance on the front end of the drive shaft is somewhat normal. Don't remember anyone having a problem at that point... The pinion bearing goes in to the housing FIRST so you have to pull both rear wheels, drain the pumpkin, pull both axle shafts, remove the driveshaft, THEN pull out the pinion. Bearing then is replaced on the pinion and reinstalled. You have to get the preload on the bearing right, and shims on the bevel gear right. As long as its apart, most people would change the two bevel gear bearings also....All can be done in a couple days time, but not by beginner.

 

The noise is only there when you are above 30 AND have your foot on the gas?

 

Yes, I need to be about approximately 30mph. If I touch the gas even a little I get the noise... As soon as my foot comes off, it goes away.

Initially I thought u-joints since it seems that putting torque through the driveshaft causes them to rub together and make the noise but I'm not sure since the noise doesn't sound like what most bad u joints sound like...

I think I'll hang a go pro under the truck and try to diagnose.

 

Just an update - the go-pro revealed noise coming from the rear pumpkin. Ended up taking it to a mechanic and it turns out to have been some chewed up bearings inside the LSD as a result of neglected maintenance... Getting a rebuilt one put in.

Thanks for the help!